What unit size is most advantageous to the AI?

[gollum]

About mercs: you are absolutely right - but i forgot to mention it because i seldom play with mercs for the last few years. Mercs in MTW are an exploit for the player beyond any measure - you can win the game well before the AI has any chance to stop you by using them. So i'd say either use them very sparingly or not at all - no matter what size you play on.

About your other point: The AI will never do more rebuilding even if you play in normal, because it usually fills all the econimic space he has with early full stacks in the beginning of the game, and unless he is given the chance to expand (which is not the norm for most AI factions) he doesnt ge the chance to lose them in battle and so to rebuild them, hopefully by better troops. In these terms all the rubble units, Slav warriors, the many javelins from VI onwards, the woodsmen etc that are available at fort level, hurt irreparably the AI factions because the AI recruits cheap units first and foremost. This means that early stacks will be full of such rubble units - and in many occasions these early stacks will be the only stacks the faction will get.

But this is not a matter of size of units - it is a matter of roster and stats optimisation in regards to the SP game.

Upgrades also hurt trmendously the AI factions and benefit the player for similar reasons - they make whatever disparities that exist between early and late era units even wider statistically and again it is the player that ripes the fruit of those and not the AI.

All in all - the problem you refer to, is solved imo with:
1. Making all units valuable (no matter how low)
2. Get rid of rubble units
3. Closing base statistical gaps of units between cultures, factions and eras
4. Abolishing weapon and armor upgrades
5. Taking (or almost taking) out mercenaries